1. Field of the Invention
An optical waveguide fiber is disclosed herein for high capacity telecommunications systems and particularly an optical waveguide fiber combining large effective area and resistance to bend induced attenuation.
2. Technical Background
Optical waveguide fibers designed for transmission of greater information capacity over long distances, preferably without use of electronic regenerators, typically reduce certain types of non-linear interactions of the signal by providing high effective area. In addition, the signal degrading effect commonly called four wave or four photon mixing, an effect that occurs in communications systems using wavelength division signal multiplexing, can be counteracted by control of the optical waveguide fiber total dispersion over the operating wavelength range. That is, the total dispersion is made to be non-zero over the operating wavelength range, thus altering the phase relationship among the signals in such a way that they do not interfere.
Through use of dispersion compensation strategies, a high capacity optical waveguide fiber can have a greater total dispersion magnitude over the operating window of a communication system. Thus, the design limitations are loosened somewhat, allowing a refractive index profile researcher to relax total dispersion requirements while improving other key fiber properties such as attenuation and resistance to bend induced attenuation.
An additional important factor in refractive index profile design of high capacity optical waveguide fibers is the simplicity of the profile as simplicity of design relates to manufacturing cost. For example, a core region that provides the desired properties but has fewer significant changes in refractive index along a radius will in general be easier to manufacture.
The present invention addresses the need for high capacity optical waveguide fiber designs which have a simpler refractive index profile structure and provide high effective area while maintaining low attenuation and providing excellent resistance to bend induced attenuation.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index (percent) and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion, sometimes called chromatic dispersion, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows. Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
Aeff=2xcfx80(∫E2 r dr)2/(∫E4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region. In those cases in which the refractive index of a segment is less than the average refractive index of the cladding region, the relative index percent is negative and is calculated at the point at which the relative index in most negative unless otherwise specified. A positive relative index percent occurs where the refractive index is greater than the average refractive index of the cladding.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b) %, where b is radius, which follows the equation, xcex94(b) %=xcex94(bo)(1xe2x88x92[|bxe2x88x92b0|/(b1xe2x88x92bo)]xcex1), where bo is the point at which xcex94(b) % is maximum, b1 is the point at which xcex94(b) % is zero, and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. Bend induced attenuation is also called bend loss herein. A bend test referenced herein is the pin array bend test that is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven in a serpentine path through the pin array and attenuation again measured. The loss induced by bending is the difference between these two measured attenuation values expressed in dB. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the serpentine woven waveguide fiber conform to the portions of the pin surface at which there is contact between fiber and pin.
Another bend test referenced herein is the lateral load wire mesh test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation is measured and expressed in dB/m. This increase in attenuation is the lateral load attenuation (or lateral load bend loss) of the waveguide.
A further test of the bend resistance of a waveguide fiber is one in which the fiber is wrapped a specified number of turns about a mandrel of a specified diameter. In each test condition the bend induced attenuation is expressed in units of dB/m, the length being determined by the number of turns of fiber and the mandrel diameter. The mandrel wrap test referenced herein is one in which induced attenuation is measured for 1 turn of waveguide fiber around a 20 mm diameter mandrel.
In one aspect, an optical waveguide fiber is disclosed herein which includes a central core region surrounded by and in contact with a clad layer. The central core region has a refractive index profile, a radius, and a centerline. The central core region has a portion with a refractive index profile configured to provide a local minimum relative refractive index percent on or near the centerline which is a fraction of the maximum relative refractive index percent of the central core region. In particular, the fraction formed by the ratio of the local minimum relative refractive index percent on or near centerline to the maximum value of relative refractive index percent in the central core region is in the range from 0.65 to 1.0. This fraction, together with the value of central core radius and maximum relative refractive index percent are chosen to provide an optical waveguide fiber having an effective area not less than 115 xcexcm2 at 1550 nm, a 20 mm mandrel wrap bend loss at 1550 nm not greater than 25 dB/m, and a lateral load wire mesh bend loss at 1550 nm not greater than 1.5 dB/m, preferably not greater than 0.5 dB/m. Advantageously, the pin array bend loss at 1550 nm is not greater than 1 dB/m. The 20 mm mandrel wrap bend loss is preferably not greater than 20 dB/m, and more preferably not greater than 10 dB/m.
The fraction preferably lies in the range from 0.75 to 0.85.
In an embodiment of the optical waveguide fiber disclosed herein, the refractive index profile parameters are selected to further provide an attenuation at 1550 nm less than or equal to 0.22 dB/km, zero dispersion wavelength no greater than 1400 nm, polarization mode dispersion not greater than 0.06 ps/km1/2, and cabled cut off wavelength no greater than 1500 nm. The attenuation at 1550 nm is preferably less than 0.20 dB/km, more preferably less than 0.19 dB/km.
In a further embodiment of the optical waveguide fiber disclosed herein, the maximum relative refractive index percent of the central core region is reached at a radius not less than 0.25 of the central core radius. The central core radius of this embodiment has a range from 6 xcexcm to 9 xcexcm and preferably a range from 6.5 xcexcm to 7.5 xcexcm.
In this first aspect, the maximum value of relative refractive index percent has a range from 0.25% to 0.45% and preferably a range from 0.28% to 0.35%.
In another embodiment of this first aspect, the optical waveguide fiber disclosed herein has a central core region that exhibits a relative refractive index percent that rises monotonically from its centerline value to its maximum value. The local minimum relative refractive index percent on or near the centerline in the central core region in this embodiment has a range from 0.2% to 0.3%.
In a second aspect, the optical waveguide fiber disclosed herein includes a central core region and an annular region of negative relative refractive index percent located between the central core region and the surrounding clad layer. Preferably, tne clad layer is adjacent to the annular region, and the annular region is adjacent to the central core region. The negative relative refractive index percent of the annular region can be achieved by adding an index reducing dopant to the annular region or by adding an index increasing dopant to the clad layer. These alternatives are in accord with the definition of negative relative refractive index percent stated above. Preferably, the central core region has a radius in the range from 7 xcexcm to 9.5 xcexcm. Also, preferably the annular negative relative refractive index percent region has inner radius equal to the central core radius, an outside radius in the range from 14 xcexcm to 18 xcexcm, and a minimum relative refractive index percent in the range from xe2x88x920.05% to xe2x88x920.15%.
The effective area is not less than 120 xcexcm, preferably is not less than 130 xcexcm, more preferably not less than 140 xcexcm2, and most preferably not less than 150 xcexcm2. In addition, the bend resistance is such that one turn of the fiber about a 20 mm diameter mandrel induces an attenuation at 1550 nm of less than 25 dB/m, and preferably less than 20 dB/m, and more preferably less than 10 dB/m.
In each of the embodiments set forth above, the OHxe2x88x92 content of the optical waveguide fiber is preferably controlled to a value sufficiently low to enable operation of the waveguide in a wavelength region including the range 1380 nm to 1390 nm.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.